Surgical reconstruction of bone defects is becoming increasingly common. Reconstruction may be needed due to tooth loss, infections, trauma, congenital defects, tumors, malignant diseases such as cancer, periodontal disease, or for a multitude of other reasons. In specific instances, the bone defect may be located in a patient's oral cavity or in the maxillary, mandibular, or palatine bones.
Some surgical techniques seek to encourage bone growth (“osteogenesis”) as well as reconstruct the bone. For example, encouraging new bone growth may help provide a fixation point for a permanent implant (e.g., in order to fix a dental implant in place), or it may be necessary when the defect is too large to repair with fixation. This is especially true in the context of oral, facial, and maxillofacial surgery. Applied to reconstruction of the mandible or maxilla, bone grafting can allow osteogenesis to occur across a gap that would not otherwise be bridged by new bone.
Bone graft materials have been used to attempt to establish new bone in a bony defect area of the body. Non-limiting examples of such bone graft materials include autologous bone, autologous bone particulate, allogenic bone graft material, human cadaver bone, xenograft bone graft material, animal bone, or synthetic materials such as hydroxyapatite, tricalcium phosphate, bioactive glass, growth factors and others. Often the patient's blood or autologous bone particles are mixed with cadaver, animal, or synthetic materials to accelerate the healing process. This technique is designed to encourage the body's normal bone healing process to extend from existing viable bone through the material and result in new bone in the area of the graft material. However, adjacent fibrous or soft tissue will often attempt to heal or migrate into the area of the graft material. In some situations, and especially in larger graft areas having direct contact with fibrous or soft tissue, the resulting healed tissue will be fibrous tissue rather than bone tissue, because fibrous tissue invades and heals more quickly than bone.
To address these problems, some surgeons have placed a complete barrier material over the bone graft material before covering it with the overlying fibrous or soft tissue in order to prevent the ingrowth of fibrous tissue. Such barriers are intended to exclude cells and newly growing blood vessels, allowing the bone graft material to heal from the areas where it contacts bone, thus encouraging only bone healing and excluding fibrous or soft tissue healing from the site. For example, one material that has been used for this purpose is expanded polytetrafluroethylene (ePTFE) which serves as a cell barrier, having a pore size in the range of about 20 microns in diameter. This membrane typically must be removed after a few months in an additional surgical procedure.
A specific commercially available material that provides a cell barrier for periodontal tissue regeneration is the GORE-TEX® Regenerative Membrane (W.L. Gore and Associates, Newark, Del.). This periodontal material is made of ePTFE and is used to provide a cell barrier between the gingiva and a periodontal defect. It is intended to preserve the necessary space between the surface of the defect and the desired contours of the subsequently regenerated surface.
It is believed that GORE-TEX®'s pore size is about 20 microns, which may allow blood infiltration and some invasion of fibrous connective tissue, but this pore size is not large enough to allow vascular infiltration (e.g., fibrovascular tissue growth into and through the material). Blood vessels are not able to penetrate the GORE-TEX® material and thus, do not provide a vascular supply to the bone graft material. Such barrier materials are desired for certain bone grafts in order to exclude fibrovascular tissue from the bone healing site, and to allow bone to heal from the edges of the bony defect into the bone graft material.
More recently, resorbable membrane materials have also been used, which do not always have to be removed. One commercially available example is VICRYL® Periodontal Mesh from Johnson & Johnson, made of woven fibers of a bioabsorbable copolymer (about 90% glycolide and 10% lactide). VICRYL® mesh is similar to a fabric. One of its drawbacks is that it does not have the stiffness to maintain a specific shape. It can also cause undesirable hydrolysis or an inflammatory reaction during the process of being resorbed. This material has not enjoyed widespread use as a bone graft material containment system. Yet another drawback for bioresorbable materials is that acid generated by resorbable materials during degradation inhibits bone growth.
Thus, although using such a resorbable material may eliminate the need for a second surgical procedure, one general problem that may be experienced is an inflammatory reaction that is a necessary part of the resorption or degradation process associated with resorbable materials. Although a resorbable material does not need to be surgically removed, the body still has to remove it by hydrolysis or by metabolizing it, and this can cause problems.
Some procedures have used protein and/or growth factors, such as one or more bone morphogenic proteins (BMP), and bone graft material at a certain surgical site. In some instances, the protein and/or growth factor is deposited on a collagen matrix with a sponge-like quality. The protein and/or growth factor material adsorbs to the collagen sponge material. The collagen sponge is then placed in a site in the patient where the growth of additional bone is desired, either alone or mixed with another type of bone growth material, such as an allogeneic, autogenous, xenograft, alloplastic, or synthetic matrix.
Specifically, the implant material containing a growth factor is used to reconstruct areas where new bone growth is desired. For example, if the growth factor is BMP, the BMP attracts stem cells and induces them to convert to osteoblasts to make new bone. It is thus not necessary to exclude the invasion of fibrous or vascular tissue into the site, because the BMP will recruit stem cells from the soft tissue sites and convert them to bone. A vascular blood supply from the surrounding soft tissue is beneficial to the newly growing bone, so a membrane that excludes cells and vascular ingrowth is undesirable (and even potentially detrimental) to the process of growing new bone. If a barrier such as expanded (e)PTFE is used, it will not allow vascular access to the site from the surrounding tissues, and it must be removed at a later date, disrupting any peripheral vascularization that is supporting the new bone.
Many of the bone graft materials mentioned above are rigid and may have adequate compressive strength to resist collapse due to pressure from the overlying tissues and scar contracture during the healing process. However, other materials such as a collagen sponge treated with a protein and/or growth factor may not be resistant to compression, and will not adequately maintain the space for bone reconstruction unless protected by a supporting structure. Adequate protection may exist inside a tooth socket, but in the case of missing bone around the socket or missing portions of the alveolar ridge, a support structure is needed to keep the collagen sponge from collapsing. In short, these grafts, although inductive in nature, are not provided in forms that will maintain a desired shape. In addition, even bone graft materials with adequate compressive strength may be displaced by the forces exerted by the overlying tissue, or by forces from adjacent mobile structures such as the lips or tongue. Thus, there is a need for a space-maintaining support structure that does not require a second surgery for removal from the tissue.
Some surgeons use metal mesh, such as titanium mesh, for this purpose. They may also add autologous or cadaver materials, such as pieces of lamellar bone, to contain the collagen sponge or bone graft material and maintain the space for bone regeneration. Soft flexible membranes such as ePTFE are generally not stiff enough to accomplish this support function. Nor are resorbable membranes. One advantage of a titanium mesh is that it has relatively large openings to allow for vascular access to the implant material from the surrounding soft tissue. However, titanium mesh is somewhat bulky and has relatively sharp edges, which risks gingival irritation and eventual erosion through the overlying soft tissue and exposure to the oral cavity. Such exposure can be a nidus for the establishment of infective agents. Bone graft material may also extend through or migrate out of some of the mesh openings. Titanium mesh may also lack flexibility during surgery and it can be difficult for the surgeon to modify its size and shape during a procedure.
Even without exposure, the titanium mesh is often felt by the patient as an irregular surface beneath the gum tissue. It can also result in an unnatural appearance of the overlying tissue, because the pattern of the mesh may be visible or palpable under the overlying soft tissue. Also, the mesh is typically dark in color or anodized to have a bright or dark surface color, making it more visible through the overlying tissue. Therefore the mesh is often removed after the bone healing takes place, resulting in an additional surgical procedure, which increases costs and patient discomfort.
Other attempts to encourage bone growth during reconstruction have used bone induction trays, such as those described in U.S. Pat. No. 3,849,805 to Leake et al. and U.S. Pat. No. 4,636,215 to Schwartz. One of the problems with the Leake and Schwartz trays is that the voids or apertures that penetrate the trays are large. If this type of tray were to be used with certain implant materials, the tray may not sufficiently contain the material. Moreover, the thickness of the trays is necessary in order to support the mandible in use, but that resulting thickness may not be efficient enough at allowing vascular access to the bone graft material. The voids or apertures may also present the irregular surface and unnatural appearance problems described above.
In addition, the large flat surfaces and large open spaces associated with metal mesh may allow overlying tissues to move relative to the fixed mesh covering the bone graft site. Such movement disrupts the formation and penetration of new blood vessels into the bone graft site, and may cause tissue breakdown resulting in exposure of the metal mesh to outside contamination. Said movement may also lead to the formation of scar tissue at the tissue-metal mesh interface, reducing vascular access to the bone graft.
In a similar problem situation, soft and flexible materials such as GORE-TEX®, may allow the overlying tissues to move relative to the graft surface, resulting in tissue breakdown or excessive scar formation over the grafted site.
It is well known that many cell types do not express their phenotype or proliferate unless they have a surface to attach to and to grow on. When encouraging bone graft materials to stimulate bone growth, vascular tissue invading the bone graft is desirable to provide a blood supply to the bone graft. Thus a porous structure is desirable that has greater surface area than is provided by direct through holes in a containment material. A porous structure with a relatively random, omnidirectional interconnected pore structure, and which still maintains openings large enough to allow formation and passage of blood vessels through the pores and into the graft site, will provide greater surface area for the ingrowth of blood vessels and for the migration of bone forming cells into the site of desired bone formation. Such a relatively high surface area structure also helps to immobilize the overlying tissue due to the greater contact between the tissue and the structures surfaces and by initial ingrowth that integrates the overlying tissue into the many varied surfaces and openings presented by the material.
In short, previous attempts for bone grafting addressed only certain types of bone graft materials, and did not seek to provide solutions that could adequately contain and support a bone graft material that does not maintain its shape (for example, a sponge treated with a protein and/or growth factor, or a particulate bone graft material that is not placed in a protected area such as inside an extracted tooth socket). Additionally, for traditional bone grafts, the prior art methods used either a barrier-type soft and flexible membrane that does not allow vascular ingrowth (in order to isolate the bone graft material from the surrounding soft tissues) or a metal or alloplastic mesh or tray with large openings (which may allow the bone graft material to migrate out of the openings, may allow movement of the overlying tissue resulting in reduced neovascularization and greater scar formation, may present comfort and appearance problems including visibility through the overlying tissues once healing has taken place, may require a second surgery for removal, and are not as efficient at allowing or conducting vascular access to the graft site).
There is thus a need for a structure having a pore size sufficient to allow fibrovascular ingrowth, but small enough to contain bone graft materials in place in the structure, against the area where bone growth is to be encouraged. It is desirable for such a design to allow vascular ingrowth into the bone graft material, while maintaining the shape and space into which the bone graft material is placed. It is also desirable for such a material to be substantially or relatively smooth on the outside surface in order to allow soft tissue to be overlaid without irritation or tissue erosion and without the bone graft material containment structure being seen under the tissue once healed. It is also desirable for such a material to have an omnidirectional and/or multidimensional open pore structure large enough to provide for growth and penetration of new blood vessels, and with greater surface area than provided by straight through holes or openings in the containment structure. It is also desirable for such a material to be rigid enough to prevent collapse or displacement of a growth factor-treated collagen sponge alone or combined with other bone graft material. It is also desirable for such a material to be rigid enough to prevent collapse or displacement of a particulate bone graft material placed inside the containment structure due to tension from the overlying soft tissue closure or scar contraction. It is desirable for such a material to be rigid enough to prevent collapse or displacement of a bone graft material due to movement of adjacent structures such as the lips and tongue. It is also desirable that the implant does not have an adverse effect on the bone growth.